[clang-format] Fix a bug in `git-clang-format --binary` (#74293)

This is a rework of #74176, which erroneously changed the default
clang-format filename (`clang-format`, `clang-format.exe`, etc.) to an
absolute pathname. Instead, we should do that only if the name is a
pathname, e.g. `./clang-format`,
`llvm-project/build/bin/clang-format.exe`, etc. See also
https://github.com/llvm/llvm-project/pull/74176#issuecomment-1837921351.